Accretion disks mediate the flow of gas and angular momentum from molecular clouds
down to the protostar. When the infall rate becomes large, the disk can no longer
transport all of the material onto the central star, causing its mass to increase.
When they become too massive, disks can fragment. I describe the role of disks in
the formation of binary and multiple systems, and the subsequent evolution of their
orbital parameters. Using global numerical experiments, I show that we can
characterize the onset of instability and fragmentation as a function of the infall
rate. Finally, I discuss the implications of disk instability for the upper mass
limit of stars, and for the formation of massive, wide-orbit planets.